The present invention relates to controlling the motion of sheets moving through a printing apparatus, where the apparatus is capable of xe2x80x9cduplexing,xe2x80x9d or printing on both sides of a sheet.
On-demand page printers, wherein images are created in response to digital image data submitted to the printing apparatus, are familiar in many offices. Such printers create images on sheets typically using electrostatographic or ink-jet printing techniques.
Particularly with sophisticated printing apparatus, it may often be desired to print xe2x80x9cduplexxe2x80x9d prints, that is prints having images on both sides of the sheet. In order to obtain duplex prints, it is common to provide a xe2x80x9cduplex loopxe2x80x9d or xe2x80x9cinverterxe2x80x9d within the printing apparatus. The purpose of an inverter is to handle a sheet after one side thereof has received an image, and in effect turn the sheet over; a duplex loop re-feeds the sheet to the image-making portion of the printing apparatus so that the individual sheet becomes available to the image-making portion twice, once for each side.
U.S. Pat. No. 5,504,568 discloses a duplex loop in a xerographic printing apparatus; the patent is generally directed toward scheduling the printing of first and second side images on each sheet.
U.S. Pat. No. 6,199,858 discloses an office printer in which sheets being readied for re-feeding to the marking station for duplexing are shingled along a duplex path.
U.S. Pat. No. 6,322,069 discloses a control system for governing the distance between sheets in a xerographic printer capable of duplexing.
According to the present invention, there is provided, in a printing apparatus having a marking station operatively disposed along a path, an inverter downstream of the marking station along the path, a duplex loop connecting the inverter to a location upstream of the marking station along the path, and a duplex sensor disposed at a predetermined location along the duplex loop for detecting a sheet, a method for controlling motion of a sheet in the apparatus. A sheet moves through the path at a process velocity and moves through a portion of the duplex loop at a duplex velocity. An error of a sheet arriving at the duplex sensor relative to a predetermined arrival time is determined. Based on at least the error, a time for initiating a change in velocity of the sheet from the duplex velocity to the process velocity is calculated.